Efficient functioning of any anode in a cathodic protection system depends on the backfill material used at the interface between the anode and the electrolyte. Normally in the cathodic protection of underground structures and pipelines buried in soil, chemical backfills in powder form are used. However, this type of backfill is not suitable for use in concrete structures. Chemical backfills can be helpful in adsorbing/absorbing soil moisture to keep the environment immediately surrounding the anode continuously moist, thereby promoting better anode current efficiency.
In the case of impressed current anode systems, carbonaceous backfills are widely used. A conductive overlay based on coke breeze has been used for cathodic protection of reinforced concrete structures. More recently, hydrogel systems have been experimented. However an efficient ion conductive backfill, notably proton conducting backfill, in sheet form based on polymer has hitherto been unknown in the prior art.
Reference is made to ‘Control of Pipeline Corrosion’ edited by A. W. Peabody and published by National Association of Corrosion Engineers, USA, 1967, page No 107; where in use of carbonaceous backfill based on coal coke breeze, calcined petroleum coke breeze and natural or man made graphite backfills is mentioned. The major drawback is that this material is in powder form and must be tamped solidly around the anode. When the contact pressure cannot be made sufficiently solid initially, or relaxes with time, much of the current will discharge directly from anode to electrolyte, thereby reducing the anode life. Petroleum Coke must be calcined to remove all other petroleum products, otherwise its resistivity will be too high. On page 120 of the document, the composition of chemical backfills for galvanic anodes has been given. Anode resistivity very mush depends on the composition. A backfill composed of 75% hydrated graphite, 20% bentonite clay and 5% sodium sulphate is mentioned. The main drawback of this backfill is that being in powder form, it is rather difficult to prevent leaching in soil in spite of confining the backfill in a bag. Further such backfills cannot be retained easily on a concrete surface. Intimate contact cannot be ensured at all seasons particularly during summer.
Reference is made to Jack Bennett and Clim Fir Lotte, Materials Performance Volume 36 March 1997, pp 14-20; wherein adhesive type of acrylic hydrogel was evaluated for use in reinforced concrete structure. The main drawback of this hydrogel is that the gel must be prevented from direct contact with water or seawater by caulking the edge. This is not practicable. It is often difficult to prevent exchange of water with the environment. Their studies further reveal that the developed hydrogel system was best suited for use with pure zinc as sacrificial anode, and that hydrogel adhesives available for medical applications are not satisfactory for cathodic protection of reinforced concrete structures. Reference is also made to R. J. Kassler R. G. Powers and I. R. Lasa, Materials Performance Vol 37, January 1998, pp 12-16; wherein performance of zinc sheet anodes with another vinyl type sheet material with an adhesive compound on both sides is reported. The main draw back of this system is that there is wide variation in performance. Another limitation is that performance of this vinyl type, sheet material system was evaluated only on zinc sheet anode system.
Hydrogels hitherto used for cathodic protection of concrete structure involve the use of classical polymer gels mainly of simple vinyl system as backfiller whose inherent water contact in the gelated physico-chemical structure has provided necessary ionic conducting pathways in the electrochemical cell configuration providing the protective system. Further, this requires coverage on both sides to prevent transport of moisture laterally.
Literature search provides the references given above. No patent was located on the use of ion conducting polymer for use in cathodic protection of concrete structures.